Increasing demand for high octane gasoline blended with lower aliphatic alkyl ethers such as octane boosters and supplementary fuels has created a significant demand for isoalkylethers, especially the C.sub.5 to C.sub.7 methyl, ethyl and isopropyl-t-alkyl ethers, such as methyl t-butyl ether, ethyl t-butyl ether, t-amyl methyl ether and t-amyl ethyl ether. Consequently, there is an increasing demand for the corresponding isoalkene starting materials such as isobutene, isoamylenes and isohexenes.
To obtain isoolefins, it is desirable to convert an alkene such as normal butene, to a methyl branched alkene, for example isobutylene, by mechanisms such as structural isomerization. Such converted isoalkenes then can be reacted further, such as by polymerization, etherification or oxidation, to form useful products. Normal alkenes containing four carbon atoms (1-butene, trans-2-butene and cis-2-butene) and five carbon atoms (1-pentene, trans-2-pentene, and cis-2-pentene) are relatively inexpensive starting compounds. Conventionally, butenes and amylenes, including to a minor extent isobutylene and isoamylene, are obtained as a by-product from refinery and petrochemical processes such as catalytic and thermal cracking units. Butenes are also conveniently obtained from butadiene via selective hydrogenation.
Zeolite materials, both natural and synthetic, are known to have catalytic properties for many hydrocarbon processes. Zeolites typically are ordered porous crystalline aluminosilicates having a definite structure with cavities interconnected by channels. The cavities and channels throughout the crystalline material generally can be of such a size to allow selective separation of hydrocarbons. Such a hydrocarbon separation by the crystalline aluminosilicates essentially depends on discrimination between molecular dimensions. Consequently, these materials in many instances are known in the art as "molecular sieves" and are used, in addition to catalytic properties, for certain selective adsorptive processes. Zeolite molecular sieves are discussed in great detail in D. W. Breck, Zeolite Molecular Sieves, Robert E. Krieger Publishing Company, Malabar, Fla. (1984).
Generally, the term "zeolite" includes a wide variety of both natural and synthetic positive ion-containing crystalline aluminosilicate materials, including molecular sieves. They generally are characterized as crystalline aluminosilicates which comprise networks of SiO.sub.4 and AlO.sub.4 tetrahedra in which silicon and aluminum atoms are cross-linked in a three-dimensional framework by sharing of oxygen atoms. This framework structure contains channels or interconnected voids that are occupied by cations, such as sodium, potassium, ammonium, hydrogen, magnesium, calcium, and water molecules. The water may be removed reversibly, such as by heating, which leaves a crystalline host structure available for catalytic activity. The term "zeolite" in this specification is not limited to crystalline aluminosilicates. The term as used herein also includes silicoaluminophosphates (SAPO), metal integrated aluminophosphates (MeAPO and ELAPO), metal integrated silicoaluminophosphates (MeAPSO and ELAPSO). The MeAPO, MeAPSO, ELAPO, and ELAPSO families have additional elements included in their framework. For example, Me represents the elements Co, Fe, Mg, Mn, or Zn, and El represents the elements Li, Be, Ga, Ge, As, or Ti. An alternative definition would be "zeolitic type molecular sieve" to encompass the materials useful for this invention.
Developments in the art have resulted in formation of many synthetic zeolitic crystalline materials. Crystalline aluminosilicates are the most prevalent and, as described in the patent literature and in the published journals, are designated by letters or other convenient symbols. Various zeolites which have been specifically named and described are, for example, Zeolite A (U.S. Pat. No. 2,882,243), Zeolite X (U.S. Pat. No. 2,882,244), Zeolite Y (U.S. Pat. No. 3,130,007), Zeolite ZSM-5 (U.S. Pat. No. 3,702,886), Zeolite ZSM-11 (U.S. Pat. No. 3,709,979), Zeolite ZSM-12 (U.S. Pat. No. 3,832,449), Zeolite ZSM-23 (U.S. Pat. No. 4,076,842), Zeolite ZSM-35 (U.S. Pat. Nos. 4,016,245 and 5,190,736), Zeolite ZSM-48 (U.S. Pat. No. 4,375,573), Zeolite NU-1 (U.S. Pat. No. 4,060,590) and others. Various ferrierite zeolites including the hydrogen form of ferrierite, are described in U.S. Pat. Nos. 3,933,974, 4,000,248 and 4,942,027 and patents cited therein. SAPO-type catalysts are described in U.S. Pat. No. 4,440,871. MeAPO type catalysts are described in U.S. Pat. Nos. 4,544,143 and 4,567,029; ELAPO catalysts are described in U.S. Pat. No. 4,500,651, and ELAPSO catalysts are described in European Patent Application 159,624.
Two general classes of catalysts have been disclosed as particularly useful for isomerizing a linear olefin to the corresponding methyl branched isoolefin. These include the porous, non-crystalline, refractory oxide-based catalysts and the zeolitic-based catalysts.
Examples of the porous non-crystalline, refractory oxide-based catalysts are aluminum oxides, such as gamma or eta Al.sub.2 O.sub.3, halogenated aluminum oxides, aluminum oxides reacted with silicon, boron or zirconium, various phosphates and solid phosphoric acids. Examples of these catalysts are described in U.S. Pat. Nos. 5,043,523, 3,531,542, 3,381,052, 3,444,096, 4,038,337, 3,663,453, British Patent No. 2,060,424 and in an article by V. R. Choudhary and L. K. Doraiswamy, "Isomerization of n-Butene to Isobutene, I. Selection of Catalyst by Group Screening," Journal of Catalysis, volume 23, pages 54-60, 1971. Illustrative of the porous, non-crystalline refractory oxide catalysts are those described in U.S. Pat. No. 4,434,315, issued Feb. 28, 1984, which discloses as a catalyst a porous alumina acidified with a critical amount of silica and containing 5 ppm to 2% by weight of palladium, chromium, nickel, copper, manganese or silver by impregnation. The use of the listed metals is said to result in a more facile catalyst regeneration. All of these catalysts deactivate rapidly. According to the examples in British Patent No., 2,060,424, run life can be as short as 1 to 2 hours. Often, it is necessary to add steam and halogen compounds to prolong the catalysts run life. German Specification No. 3,000,650-A states that the run life can be increased to approximately 50 hours by these methods although this is still less than desirable.
With regard to the zeolitic-based catalysts, the most significant use has involved the large pore zeolites or those having two or more-dimensional interconnecting channels. Examples of the zeolitic-based catalysts having two or more-dimensional interconnecting channels used in association with catalytic metals are U.S. Pat. No. 4,435,311 (with platinum and palladium) and U.S. Pat. Nos. 4,503,282 and 5,227,569 (impregnated or ion-exchanged with metals including Group VIII). Examples of the large pore zeolitic-based catalysts used in association with catalytic metals are U.S. Pat. No. 5,227,569 (impregnated or ion-exchanged with metals including Group VIII) and U.S. Pat. No. 4,392,003 (with gallium).
More recently, European Patent Publication Number 523,838 A2, published Jan. 20, 1993, has disclosed a process for structurally isomerizing a linear olefin to its corresponding methyl branched isoolefin using as a catalyst a zeolite with one or more one-dimensional pore structure having a pore size small enough to retard by-product dimerization and coke formation within the pore structure. and large enough to permit entry of the linear olefin and allow formation of the methyl branched isoolefin. It has been found that as these small pore catalysts are used, they acquire a build-up of coke which diminishes their effectiveness. To restore their effectiveness, the catalysts must be regenerated at elevated temperatures by contact with oxygen. This regeneration process, when repeated a number of times, can have an adverse effect on the catalyst life and selectivity.
A typical zeolitic catalyst regeneration temperature is described in "Chemistry Of Catalytic Processes", by B. C. Gates, J. R. Katzer and G. C. A. Schuit, McGraw-Hill Book Company, New York (1979) at pages 1-5 as between a temperature of 650.degree. C. to 760.degree. C. A recent trend is toward higher regeneration temperatures. For example, a regeneration temperature as high as 850.degree. C. is used in the commercial regeneration of zeolitic catalysts used in Fluid Catalytic Cracking ("FCC"). J. Biswas and I. E. Maxwell, Applied Catalysis, 63 (1990), 197-258.
However, it has been found that use of such high regeneration temperatures such as those used in FCC results in poor olefin isomerization performance (lower selectivity) for a medium pore-sized zeolite-based catalyst such as those described in European Patent Publication Number 523,838. According to U.S. Pat. No. 5,043,523, a regeneration temperature of 550.degree. C. to 600.degree. C. is recommended for a modified alumina catalyst of the type discussed earlier. The modified alumina catalyst was reported to show no signs of deactivation after undergoing 10 regeneration cycles at 575.degree. C. by method A of Example 29. However, it has been found that zeolitic catalysts with one or more one-dimensional pore structure having a pore size small enough to retard by-product dimerization and coke formation within the pore structure and large enough to permit entry of the linear olefin and allow formation of the methyl branched isoolefin, such as ferrierite, ZSM-22 and ZSM-23 tend to lose selectivity for the formation of isoolefins when exposed to temperatures of greater than 565.degree. C. for a period of time such as those used in the regeneration processes mentioned above.
Commercialization of an isomerization process to manufacture isoolefins from linear olefins has been further hampered by longer regeneration times compared with run life.
It is therefore an object of the present invention to provide a medium pore zeolite catalyzed process for structurally isomerizing a linear olefin to its corresponding methyl branched isoolefin with improved run life and/or reduced regeneration time. It is another object of the present invention to provide a medium pore zeolite catalyzed process for structurally isomerizing a linear olefin to its corresponding methyl branched isoolefin with improved overall yield. It is further an object of the present invention to provide an improved medium pore zeolite catalyst for structurally isomerizing a linear olefin to its corresponding methyl branched isoolefin.